/**
 * 79. 单词搜索
 * https://leetcode.cn/problems/word-search/description/
 */
class Solution {
    boolean[][] vist;
    char[] word;
    int m, n;
    public boolean exist(char[][] board, String _word) {
        m = board.length;
        n = board[0].length;
        word = _word.toCharArray();
        vist = new boolean[m][n];

        for(int i = 0; i < m; i++) {
            for(int j = 0; j < n; j++) {
                if(board[i][j] == word[0]) {
                    vist[i][j] = true;
                    if(dfs(board, i, j, 1)) return true;
                    vist[i][j] = false;
                }
            }
        }
        return false;
    }
    int[] dx = {0, 0, 1, -1};
    int[] dy = {1, -1, 0, 0};
    public boolean dfs(char[][] board, int i, int j, int pos) {
        if(pos == word.length) {
            return true;
        }
        //上下左右去匹配
        //利用向量数组，一个for循环就可以搞定上下左右四个方向
        for(int k = 0; k < 4; k++) {
            int x = i + dx[k], y = j + dy[k];
            if(x >= 0 && x < m && y >= 0 && y < n && !vist[x][y] && board[x][y] == word[pos]) {
                vist[x][y] = true;
                if(dfs(board, x, y, pos + 1)) return true;
                vist[x][y] = false;
            }
        }
        return false;
    }
}